This invention is related to the power generation industry and, more particularly, to the field of electrical power generators.
In the power-generation industry, high voltage coils are often used in a stator of power generators. In inner cooled high voltage stator coils, cooling members having a tubular shape or configuration, and known as cooling tubes or vent tubes, are often used integral to or positioned within the outer confines of the coil construction. The cooling tubes are made of conductive material, e.g., a metal, and serve to pass coolant, e.g., air or hydrogen, through them to remove copper (or other conductive material) conductor heat. The tubes are usually surrounded by copper strands of the coil and extend the full length of the coil. For example, many coil designs or configurations contain five (5) or more cooling tubes in a single stack.
All of the vent tubes in a coil stack are insulated from each other and insulated from the copper strands, e.g., to prevent flow of machine or generator current in the tubes. No current from the generator is allowed to flow in the vent or cooling tubes, and the tubes are conventionally not designed to carry generator current, e.g., the tubes often have a thin-wall type construction or design. Examples of some vent or cooling tube configurations can be seen in U.S. Pat. No. 5,323,079 by Nieves et al. titled xe2x80x9cHalf-Coil Configuration For Statorxe2x80x9d and U.S. Pat. No. 5,723,920 by Markovitz et al. titled xe2x80x9cStator Bars Internally Graded With Conductive Binder Tape.xe2x80x9d
In these systems, however, and with voltage on the stator coil, a large percentage of voltage often appears between the metal vent tubes and the copper strands of the coil. In order to prevent the flow of machine or generator current in the vent tubes, both the copper strands and the vent tubes are insulated. This voltage potential, if allowed to reach high values, can destroy the copper strand insulation and the vent tube insulation which respectively surrounds the strands and the tubes as described above. Therefore, vent tube voltage grading is needed to prevent the buildup of voltage between the vent tubes and the copper strands.
The method of construction of stator coils conventionally used to address this need for voltage grading is the connection of external resistors between the copper strands and the vent tubes. On some coil designs, however, no space is available to connect the resistors externally to the coil. In addition, it is impossible or can be very difficult to make connections to the inner stack or arrangement of vent tubes and, accordingly, grading cannot be readily achieved. Even though the voltage on the inner vent tubes is much lower than the top and bottom vent tubes in a stacked configuration, some magnitude of protection is still needed for these vent tubes as well.
In view of the foregoing, the present invention advantageously provides a power generation system and associated methods having voltage grading to inner vent tubes of stator coils of a stator of a power generator. Also, the present invention advantageously provides a stator having stator coils with a compact voltage grading arrangement for effectively providing voltage grading capabilities to inner vent tubes of the coils. The present invention still also advantageously provides a power generation system, stator coils, protecting devices, and associated methods which compactly reduce voltage buildup situations between venting devices and conductive strands of coils. Further, the present invention advantageously provides a stator coil and associated methods which significantly reduces insulation failure in stator coils of a power generation system.
More particularly, the present invention provide a stator for a power generation system which preferably includes a plurality of high voltage stator coils. Each of the plurality of high voltage stator coils includes a plurality of metal strands, a plurality of vent members positioned adjacent the plurality of metal strands, and compact voltage grading means contacting each of the plurality of vent members and the plurality of metal strands for grading voltage between the vent members and the metal strands to thereby prevent an over voltage condition. For example, the metal strands and vent members are often positioned in a stacked configuration and the compact voltage grading means contact even the inner vent members of the stack.
Additionally, the present invention provides an overvoltage protector for a power generation system. The overvoltage protector preferably includes at least a first conductive strip member positioned to contact one of a plurality of vent members, a voltage grading layer of material positioned to contact the first conductive strip member, and at least a second conductive strip member positioned to contact the voltage grading layer and at least one of a plurality of conductive coil strands forming a portion of a high voltage coil.
The present invention still further provides a method of grading voltage between internal vent members and conductive strands of a high voltage coil of a power generation system. The method preferably includes connecting conductive portions of each of a plurality of internal vent members to a plurality of conductive strands of a high voltage coil. The connecting step for each vent member can advantageously include forming an opening in insulation surrounding the vent members, positioning a first conductive strip member to contact conductive portions of the vent member, positioning a voltage grading layer of material to contact and overlie the first conductive strip member, and positioning a second conductive strip member to contact the voltage grading layer and at least one of the plurality of conductive strands. The voltage grading layer preferably includes a plurality of layers of conductive tape, and the step of positioning the voltage grading layer preferably includes adhering a first layer of conductive tape to the first metal strip member and the insulation surrounding the vent member and adhering a second layer of conductive tape to the insulation and the second conductive strip member.